560 KfiPORTS ON THE STATE OP SCIENCE. — 1916. 



that the errors grew continuously and linearly with time between the 

 stations, and obtaining the corrections to be applied to any period of the 

 voyage from the graph. These are given in Table IV. under the 

 columns headed l^,, lb, and le, and the final corrected values in columns 

 -^•iga, -^2gb, and A^g^. 



In figs. .5(2), 6(2), 7(2) these values are plotted against the latitude of 

 the ship at the time of observation, the stations chosen as standards 

 being indicated by arrows at the base of the diagrams. Sydney and 

 Reading are omitted. 



The mean of the Adelaide and Fremantle Harbour observations was 

 chosen as a standard {i.e. they were made equidistant from the theoretical 

 carve) on account of the paucity of the observations at these two ports. 

 For certain reasons the Adelaide readings are the least reliable, and I now 

 believe it would have been preferable to have taken the Fremantle 

 Harbour observations as correct. 



Even when, as in fig. 5(2), only the Australian ports, Bombay, Aden, and 

 Tilbur}^ were chosen as reference ports, the intervening stations Colombo, 

 Malta, Suez Canal, and Plymouth Harbour fall close to the theoretical 

 line, which to some extent justifies the assumption made in correcting for 

 station errors ; moreover, the continuity of the dots between Australia and 

 Bombay suggests that in this region there has been no sudden change in 

 the properties of the aneroid. 



In order to see if the systematic error introduced b}^ the pumjjing of 

 the aneroid could be responsible for these low values of gravity, the devia- 

 tions from the theoretical formula were plotted against the pumping, i.e. 

 the maximum difEerence from the mean of each set of aneroid reaclings, 

 fig. 8. No general dependence is to be discerned, for though a defect of 

 gravity is usually accompanied by moderate pumping there are almost as 

 many instances of excess values under the same conditions. A further 

 examination showed that the defects in gravity between Fremantle and 

 Aden were greater (nearly twice as great) than could be explained even 

 on the assumption that the aneroid readings were too low by an amount 

 as large as the extreme measured pumping of tbat instrument. The only 

 evidence in favour of a connexion between the pumping and the defect of 

 gravity is derived from the early part of the voyage of ss. Ascanius, fig. 12, 

 when the pumping was great because the aneroid was not mounted on the 

 spring support. On this occasion, the deviation from the theoretical 

 curve was great also, but it is just as possible that this was due to the 

 instrument not having been levelled. On the other hand, the curves 

 reproduced in figs. 9 and 13, in which pumping and defect of gravity are 

 plotted together against time, show little correlation, and on the whole, 

 the evidence is against this particular instrumental detect having vitiated 

 the results ; it is possible that over-caution has been shown in labouring 

 this point ; nevertheless, it is one which must not be overlooked in the 

 design of future aneroids to be used for gravity determinations at sea. 



Thus, as far as the evidence goes, the conclusions arrived at by 

 Hecker as a residt of his investigation by means of the boiling-point 

 thermometer are not confirmed, and one may seriously doubt whether 

 Helmert's formula holds over ocean depths as closely as has been 

 supposed. 



